<map name>,<x>,<y>,<facing>%TAB%script%TAB%<NPC Name>%TAB%<sprite id>,<triggerX>,<triggerY>,{<code>}
TriggerX and triggerY, if given, will define an area, centered on NPC and
spanning triggerX cells in every direction across X and triggerY in every
direction across Y. Walking into that area will trigger the NPC. If no
'OnTouch:' special label is present in the NPC code, the execution will start
from the beginning of the script, otherwise, it will start from the 'OnTouch:'
label. Monsters can also trigger the NPC, though the label 'OnTouchNPC:' is
used in this case.
https://rathena.svn....pt_commands.txt
Trigger X and Y just declares how far the player has to be from the NPC in the x and y direction for it to trigger.
I'm not sure though how big of a number you can put in there for it to work properly.